The invention relates to a method to regulate the pressure inside a cryogenic tank containing cold gas when a main stream for a consumer is withdrawn, whereby heated gas in the form of a heating gas secondary stream, which is regulated as a function of the tank pressure, is fed through the cryogenic tank via a pipe.
Moreover, the invention relates to a device to regulate the pressure inside a cryogenic tank when at least one main stream for a consumer is withdrawn, said device having a thermally insulated reservoir to hold a cold gas that is connected to a withdrawal line that serves to withdraw a gas stream, having a device to heat up the gas stream, having a pipe that runs through the reservoir, and having a pressure-control means with which the gas stream in the form of a heating gas secondary stream, which is regulated as a function of the tank pressure, is fed through the cryogenic tank via the pipe.
A cryogenic tank is a heat-insulated reservoir that holds liquid, deep-frozen gases. The term xe2x80x9ccold gasxe2x80x9d as used here and below also refers to liquefied gases. Such cryogenic tanks are employed, for example, in automotive technology as fuel tanks. They can contain liquid hydrogen, liquid natural gas, liquid nitrogen or the like. Hydrogen, in particular, is an especially environmentally friendly fuel since only water vapor is formed when it is burned. This is why great importance is ascribed to hydrogen when it comes to future-oriented automobile concepts. In order to be liquefied, hydrogen is cooled down to a temperature below xe2x88x92253xc2x0 C. [xe2x88x92423.4xc2x0 F.] and then stored in liquid form at a slight excess pressure in such a cryogenic tank. The pressure in the cryogenic tank drops when gas or liquefied gas is withdrawn. However, in many applications such as, for instance, the above-mentioned use as a fuel tank, the pressure in the tank has to be kept as constant as possible.
For purposes of regulating the pressure when a main stream for a consumer is withdrawn from a cryogenic tank, DE-A 196 45 492 proposes a method and a device of the above-mentioned type in the form of a so-called xe2x80x9cintrinsic gas convectorxe2x80x9d whereby, in order to maintain or increase the pressure, heated gas in the form of a xe2x80x9cheating gas secondary streamxe2x80x9d is fed through a pipe that runs through the reservoir. The heat that is introduced into the cryogenic tank in this process promotes the evaporation of liquefied gas, thus increasing the tank pressure. The pressure drop in the reservoir caused by the withdrawal of the main stream can thus be equalized once again by a controlled circulation of the hot secondary stream. In the prior-art device, the heating gas secondary stream is conveyed in a closed circulation system and the throughput is regulated by a control valve as a function of the pressure in the tank. Even though the throughput of the requisite secondary stream is at a certain ratio relative to the throughput of the main stream conveyed to the consumer, it can vary over a wide range, depending on the design and operating conditions of the cryogenic tank and on the requirements of the consumer. The problem also arises that, depending on the quantity consumed, the main stream has to be throttled to different degrees, although the pressure drop between the reservoir and the consumer should remain as constant as possible. With the prior-art device and with the prior-art regulation methods, these requirements can only be met with highly complicated control technology and equipment.
The invention has the objective of creating a simple and safe operating method to regulate the main stream and the secondary stream while also providing a cost-efficient device for this purpose.
As far as the method is concerned, this objective is achieved according to the invention on the basis of the prior-art method in that the secondary stream and the main stream are fed to a pressure-control means equipped with a first adjustable throttle for the secondary stream and with a second adjustable throttle for the main stream, whereby the throttles are coupled to each other in such a way that they can be opened and closed in opposite directions as a function of the pressure in the tank.
The main stream is conveyed to a consumer. The secondary stream in the form of a heating gas stream flows through the cryogenic tank in a pipe, thus serving to raise the pressure in the tank by introducing heat. In order to regulate the throughputs of the main stream and of the secondary stream, each stream is provided with an adjustable throttle by means of which the minimum free flow cross section for the main stream and for the secondary stream can be adjusted. The two throttles are coupled to each other in such a manner that an enlargement in the flow cross section of one throttle is associated with a reduction in the flow cross section of the other throttle and vice-versa. The secondary stream is adjusted by means of the first throttle as a function of the pressure in the tank. Since the first throttle is coupled to the second throttle for the main stream, any change in the free flow cross section of the first throttle simultaneously influences the free flow cross section for the main stream. In this manner, the method according to the invention allows a very simple regulation of the main and secondary streams while the pressure in the tank is kept largely constant by a single pressure-control means. If the pressure in the tank is too low, the throttle for the secondary stream is opened or opened further and, simultaneously, the throttle for the main stream is cut back. By increasing the throughput for the secondary stream, a greater quantity of heat is introduced into the cryogenic tank, thus causing the pressure to rise. This pressure rise, in turn, causes the first throttle to close, thus correspondingly reducing the secondary stream while, at the same time, enlarging the flow cross section for the main stream.
It turned out to be particularly simple to employ an approach in which a primary gas stream from which the main stream and the secondary stream are branched off is withdrawn from the cryogenic tank. The main stream and the secondary stream are branched off from a shared primary gas stream. As a consequence, only one withdrawal line is needed. The throughputs for the main stream and for the secondary stream are regulated by the pressure-control means which, seen in the direction of flow of the primary gas stream, is arranged downstream from the branch-off site.
It has proven to be advantageous to employ a three-way pressure regulator as the pressure-control means and to feed the main stream to a first gas inlet and the secondary stream to a second gas inlet of the three-way pressure regulator. Accordingly, the three-way pressure regulator is fitted with two gas inlets and one gas outlet in the direction of the consumer, whereby the main stream and the secondary stream are conveyed to the three-way pressure regulator separately from each other. If there is a pressure gradient between the pressure tank and the three-way pressure regulator, this approach makes it possible to convey both gas streams in the direction of the pressure gradient so that auxiliary means such as, for example, blowers or pumps, are not needed in order to generate the gas flow. Such a pressure gradient can be created, for instance, by heating up the cold or liquefied gas contained in the cryogenic tank. The heating causes the gas density to decrease so that the volume and the tank pressure increase correspondingly.
Advantageously, the secondary stream is fed through the cryogenic tank, it is subsequently re-heated and then conveyed to the second gas inlet of the pressure-control means. The secondary stream cooled off in the cryogenic tank can be heated up, for example, in a heat exchanger that is kept at room temperature without the need for additional heating material. Heating up the tank contents by means of the secondary stream generates a pressure gradient so that the secondary stream flows in the direction of the pressure-control means without any auxiliary energy.
Another improvement can be achieved if the secondary streamxe2x80x94seen in the direction of flowxe2x80x94is added to the main stream in the area downstream from the first throttle. This approach entails the advantage that, when the first throttle is openxe2x80x94and the free flow cross section for the main stream is correspondingly reducedxe2x80x94the gas that flows through the first throttle can likewise be conveyed to the consumer. This makes it easier to maintain the main stream for the consumer and avoids gas losses.
In the case of one method variant in which the gas is present in the cryogenic tank in a liquid phase and in a gaseous phase, it has proven to be advantageous to withdraw the main stream for the consumer from the gaseous phase. Even though withdrawing liquid gas from the cryogenic tank requires relatively less energy to maintain the tank pressure, evaporation pulsations can occur in an evaporator located downstream. In contrast, with the method variant according to the invention, no liquid is withdrawn from the cryogenic tank, but rather cold gas. Therefore, the liquid is already evaporated in the cryogenic tank. The larger liquid volume in the cryogenic tank prevents evaporation pulsations, thus functioning as a dampening volume in this case. Therefore, in the device suitable for this purpose, the withdrawal line used to withdraw the main stream for the consumer does not protrude into the liquid phase, but rather, it opens up into the gas phase located above.
The method according to the invention has proven its worth particularly for regulating the tank pressure when liquefied hydrogen, liquefied nitrogen or liquid natural gas are employed as the gas.
As far as the device is concerned, the above-mentioned objective is achieved according to the invention in that the pressure-control means has a first adjustable throttle to regulate the secondary stream and a second adjustable throttle to regulate the main stream, whereby the throttles are coupled to each other in such a way that they are opened and closed in opposite directions as a function of the pressure in the tank.
The main stream is conveyed to a consumer. The secondary stream in the form of a heating gas stream flows through the cryogenic tank in a pipe, thus serving to raise the pressure in the tank by introducing heat. The secondary stream is regulated as a function of the pressure in the tank. In order to regulate the throughputs of the main stream and of the secondary stream, each stream is provided with a throttle of a shared pressure-control means with which the minimum free flow cross section for the main stream and for the secondary stream can be adjusted. The two throttles are coupled to each other in such a manner that an enlargement in the flow cross section of one throttle is associated with a reduction in the flow cross section of the other throttle and vice-versa. In this manner, the device according to the invention allows a regulation of the main and secondary streams while the pressure in the tank is kept largely constant by means of a single pressure-control means. If the pressure in the tank is too low, the throttle for the secondary stream is opened or opened further and, simultaneously, the throttle for the main stream is cut back. By increasing the through-put for the secondary stream, a greater quantity of heat is introduced into the cryogenic tank, thus causing the pressure to rise. This pressure rise, in turn, causes the first throttle to close, thus correspondingly reducing the secondary stream while, at the same time, enlarging the flow cross section for the main stream. This creates a self-regulating system.
It has proven to be advantageous to configure the pressure-control means as a three-way pressure regulator equipped with a gas outlet, a first gas inlet for the main stream and a second gas inlet for the secondary stream. Accordingly, the three-way pressure regulator is fitted with two gas inlets and one gas outlet in the direction of the consumer, whereby the main stream and the secondary stream are conveyed to the three-way pressure regulator separately from each other. If there is a pressure differential between the tank and the three-way pressure regulator, this embodiment of the device according to the invention makes it possible to convey both gas streams in the direction of the pressure gradient without additional auxiliary energy so that, for example, blowers or pumps, are not needed. Such a pressure gradient can be created, for instance, by introducing heat into the cryogenic tank, for example, by feeding heated gas through the cryogenic tank. The heating causes the gas density to decrease so that the volume and the tank pressure increase correspondingly.
An embodiment of the device according to the invention can be designed particularly simply and cost-effectively in that the first throttle and the second throttle are mechanically connected to each other by means of a coupling element that can move as a function of the pressure in the tank.
A suitable coupling element comprises a valve rod that can be moved along its lengthwise axis as a function of the tank pressure and on which a first valve cone and a second valve cone are held at a distance from each other, whereby the throttlesxe2x80x94each having a valve seat as the receptacle for one of the valve conesxe2x80x94are opened or closed by moving the valve rod. The valve rod is an elongated component, for instance, a cylinder, a hollow cylinder or a cone on which at least two valve cones are rigidly or moveably attached. The position of the valve rod and thus the position of the valve cone are established as a function of the tank pressure in conjunction with a pre-specified target pressure. Moving the valve rod causes one of the valve cones to move in the direction of its valve seat, thus reducing the free flow cross section of the throttle in question while the free flow cross section of the other throttle is enlarged, so that the valve seats of the first and second throttles are opened and closed in opposite directions when the valve rod is moved. The term valve cone as defined in this invention refers to a closing element that, in conjunction with the corresponding valve seat, is capable of changing the free flow cross section of a throttle. To this end, it is not necessary for the valve cone to have the shape of a cone in the geometrical sense.
Advantageously, the valve rod extends between an upper pressure chamber where the tank pressure prevails and a lower, closed pressure chamber where a pre-specified target pressure prevails and where a pressure element acts upon the valve rod. This pressure element can be a mechanical component, for example, a membrane, a spring or a bellows, or else a gaseous or liquid pressure spring. The pressure differential between the tank pressure and the target pressure determines the position of the valve rod. This pressure differential can be adjusted particularly easily in that the valve rod has a drill hole that creates a fluid connection between the upper and the lower pressure chambers, in other words, it is sealed off towards the outside.
Another improvement can be achieved when the drill hole is provided with a restricted flow zone. This restricted flow zone serves to dampen vibrations caused by pressure fluctuations.
It has proven to be especially advantageous to pass the valve rod through at least two guide elements arranged at a distance from each other. These guide elements prevent the valve rod from tilting, thus ensuring a precise axial movement of the valve cones in the direction of the lengthwise axis of the valve rod.
A particularly simple embodiment of the device according to the invention is one in which the pressure-control means has a housing with a removable top part and a bottom part, whereby one of the guide elements is configured in the top part while the other guide element is configured in the bottom part. The removable top part allows a simple assembly of the valve rod.
Advantageously, the surface of at least one of the valve cones is made completely or partially of plastic, preferably polyamide. The valve cone can consist of plastic in its entirety. However, at least in those surface areas that come into contact with the corresponding valve seat, it is provided with a plastic layer. This prevents or reduces noises that might occur due to vibrations of the valve cone in its appertaining valve seat. As an alternative or as a complement to this, the valve seat can also be provided with an appropriate plastic surface.